Process for the sweetening of hydrocarbon oils



4 July 20,V 1943. 1 J. H. MccLlNTocK f v 2,324,790 I PnccEss FOR THEswmJTENI-NGl pr* HYDRocARBoN OIL FiledApril s, 1941 v .3 onf uM 1u uns)rz ,frana cl' vez un Lann Irc Patented July 20, 1943 PROCESS FOR THESWEETENIN G OF HYDROCARBON OILS John H. McClintock, Cranford, N. J.,assignor to Standard Oil Development Company, a. corporation of DelawareApplication April 26, 1941, serial No. 390,530

(ci. v19e-ss) 2 Claims.

The present invention is concerned with the refining of mineral oils.The invention more particularly relates to a process for the removal ofundesirable sulfur compounds from petroleum oils, and is especiallyconcerned with an improvement in sweetening operations in whichmercaptan compounds are converted to harmless disuldes, utilizing analkali metal plumbite solutionand free sulfur. The invention isparticularly applicable in the treatment of petroleum oils boiling inthe motor fuel boiling range employing the so-called doctor solutioncomprising a sodium plumbite solution. In accordance with the presentprocess two stages are ernployed under conditions in which a relativelylarge amount of clean, fresh doctorV is added in the secondary stage. Apreferred modification of the process is to contact the feed oil with arelatively small critical amount of doctor solution in the initialstage, separate the oil and contact the same with a relatively largeamount of doctor solution in the secondary stage.

It is known in the art to treat mercaptan-containing mineral oils,particularly mercaptan-containing petroleum oils, in various Ways inorder to remove the mercaptan compounds or to convert them into harmlessdisuldes. This is necessary since mercaptan materials render the oilsour and prohibit its commercial use. The usual method employed in thetreatment of petroleum oils, particularly in the treatment of thepetroleum oils boiling in the motor fuel boiling range, is to contactthe oil under suitable conditions with a so-called doctor solutioncomprising an alkaline sodium plumbite solution.

The sodium plumbite reacts with the mercap-V tans to form oil-solublelead mercaptides. These mercaptides are converted to oil-soluble alkyldisuldes and the lead removed from the treated oil as precipitated leadsulfide by treatment usually with free sulfur. For example, in thepreparation of a doctor solution comprising a sodium hydroxide solutionof sodium plumbite the usual practice is to employ a sodium hydroxidesolution of less than about Baum, generally a solution having aconcentration in the range from about 12 to 14 Baume and to add theretolead oxide. 'I'he common practice is to add suiiicient lead oxide tosaturate the solution. Any

additional lead oxide added merely settles out.

This reaction may be illustrated by the following equation:

Under :these conditions when using, for example, 14 Baume sodiumhydroxide, the approxi-l mate concentration of lead oxide is 6.8 lbs.per bbl. (42 gallons) of sodium hydroxide solution. When solutions ofthis character are contacted with mercaptan-containing oils themercaptan compounds are converted to soluble lead mericaptides. Thesesoluble lead mercaptides are then reacted usually with free sulfurresulting in the formation of precipitated lead suldes and alkyldisulphides. These reactions may be illustrated as follows:

This operation is not entirely satisfactory, since the doctor solutiontends to cause emulsi- I'lcation difficulties which results in arelatively large loss of oil in the spent doctor solution. Furthermore,an appreciable amount of floccu@ lant lead sulfide remains in thetreated oil which is relatively difficult to remove. It has thereforebeen proposed that a suflicient excess `of doctor solution be employedso as to secure the basic lead mercaptide in accordance with .thefollowing reaction:

The basiclead mercaptide when treated with sulfur results in theformation of alkyl disulfldes and precipitated lead polysulde, or basiclead sulfide. This may be illustrated by the following reaction:

This reaction, however, is not entirely satisfactory for severalreasons, one of which is that a relatively large amount of doctorsolution must be employed.

I have now discovered a process by which it is possible to remove thelead sulfide carry-over efficiently and to free the treated oil of thesame completely. In accordance with my process theV treated oil removedfrom the initial stage containingthe ilocculant lead sulfide iscontacted in a secondary stage with a relatively large amount of freshdoctor solution completely free of lead sulfide, under conditions toremove the lead sul.-

fide from the treated oil. The fresh doctor solution from the secondarystage is then recycled to the initial stage wherein conditions areadapted to remove the mercaptans from the oil. A desirable modificationof my process is to treat the oil in an initial stage under conditionsto form the basic lead mercaptide. The oil in a slightly acidiccondition is removed from the initial treating zone and contacted in asecondary treating zone in contact with a relatively large volume offresh doctor solution substantially completely free of lead sulde, underconditions to convert the basic lead mercaptides to normal mercaptides.The process of my invention may be readily understood by reference tothe drawing illustrating embodiments of the same.

For the purpose of description it is assumed that the feed oil comprisesa mercaptan-contain-` ing feed oil boiling in the motor fuel boilingrange. This oil is introduced into the system by means of line l andmixed with fresh alkali metal plumbite solution which is introduced bymeans of line 2. For the purpose of description it is assumed that thealkali metal plumbite solution is a doctor solution. The mixture ispassed thro-ugh mixing zone 3 under conditions to secure suitablecontact between the respective streams. Sulfur is added by means of line29. The mixture is passed into treating and separating zone 4 wherein aseparation is made between the treated oil and the spent doctorsolution. In one adaptation of the operation treating conditions in zone4 are adjusted so as to secure the cornplete conversion of themercaptans to the corresponding alkyl disuldes. The treated oil free ofmercaptans and containing flocculant lead sulfide due to the inherentnature of the operation is withdrawn from Zone 4 by means of line I andmixed with an f additional quantity of fresh doctol solution which isintroduced by means of line 8. This fresh doctor solution issubstantially completely free of lead sulfide. The mixture is pasedthrough mixing zone 9 and introduced into Zone l wherein a separation ismade between the treated oil and the fresh doctor solution added bymeans of line 8. The treated oil completely free of flocculant leadsulfide isremoved by means of line Il' while the doctor solutioncontaining the lead sulfide is removed by means of line l2 and recycledto the fresh feed in initial treating Zone 4 by means of line i3. Byoperating as described in which a relatively large amount of freshcaustic free of lead sulfide is introduced into the secondary stage, atreated oil free of flocculant lead sulfide is removed from thesecondary stage. When operating as described the fresh doctor solutionis functioning in removing flocculant lead sulfide from thetreated oil.

A preferred operation in accordance with the present invention is toadapt the treating conditions and the respective feed rates so that thelead mercaptides are present in zone 4 as the normal lead mercaptides.Thus, when sulfur is added, the normal lead mercaptides will react withthe sulfur to precipitate lead sulde and form alkyl disuldes. This modeof operation is secured by utilizing a relatively small amount of doctorsolution in the initial treating zone. The spent doctor solutioncontaining the lead as lead sulde is withdrawn from zone 4 by means ofline 5, passed to regeneration zone 2l and recycledto sodium plumbitestorage 22 by means of line 23. The partially treated oil containingdissolved therein a relatively small amount of unremoved mercaptans iswithdrawn from zone 4 by means of line 'l and mixed with an additionalrelatively large quantity of fresh doctor solution which is introducedby means of line 8. The doctor solution is substantially completely freeof lead sulfide. The mixture is passed through mixing zone 9 whereinadditional sulfur is added by means of line 24, under conditions tosecure adequate contact between the respective streams. The mixture ispassed into zone lll in which a separation is made between the doctorsolution and the treated oil. Temperature and pressure and therespective feed rates are adjusted in zone lil so as to secure theformation of basic lead mercaptides which upon the addition of sulfurwill result in the precipitation of basic lead sulde. The spent doctorsolution containing the precipitated basic lead sulfide and thecarry-over lead sulfide from the initial treating zone is removed fromzone I0 by means of line I2 and preferably recycled with the feed oil tothe initial treating zone by means of line I3. However, under certainconditions it may be desirable to pass at least a portion of the spentdoctor solution withdrawn from the secondary treating Zone toregeneration zone 2| by means of line 25. 'Ihe treated oil is withdrawnfrom the system by means of line l I and handled as desired.

Although I do not wish to be restricted by the same it is my belief thatthe reactions involved are as follows:

Initial zone Doctor from secondary zone Added In feed fresh oil Sulfuradded- Initial zone 2Pbs+2RssR Secondary zone infied Added fresh Rs PbsSulfur added-Secondary zone In the above reactions the basic lead suldeprecipitated in the secondary Zone will more readily separate and willnot be carried over with the treated oil to any extent. Furthermore, theprecipitation of the basic lead sulfide will substantially completelyremove the carry-over ilocculant lead sulfide from the initial Zone.

The process of the present invention may be widely varied. It is to beunderstood that the respective Zones may comprise any suitable numberand arrangement of units. The invention essentially comprises employinga relatively large amount of fresh doctor solution in a secondarytreating zone in order completely to remove flocculant lead sulfide fromthe oil in said zone. Doctor solution utilized in the secondary treatingzone is then contacted with a feed oil in the initial treating zoneunder conditions to remove the mercaptans from the feed oil.

Although it is preferred completely to remove the mercaptans in theinitial treating zone, under certain conditions it may be desirable toutilize a relatively small amount of doctor solution in the initialtreating zone so that lead sulfide is PbS

